1. Field of the Invention
This invention relates to low-viscosity, pregelable rubber-based adhesives having improved performance properties, to a process for producing these new adhesives, and to their use.
2. Statement of Related Art
The use of epoxy resin or plastisol adhesives in car manufacture has long been known. Important requirements which adhesives such as these have to satisfy include firm adhesion of the adhesive material to steel surfaces and to galvanized surfaces and pregelability of the adhesive at temperatures higher than 120.degree. C.
Good results are also obtained with solvent-free rubber-based adhesives. Rubber-based adhesives such as these have to satisfy stringent demands in regard to the strength of the bond and low-temperature flexibility. In addition, materials of this type have to show low viscosity and favorable rheological properties, for example short break-off and high stability, in order to lead to stable, long-lasting bonds in car manufacturing. To achieve this, it is necessary to use solid rubbers in addition to the basic liquid rubber materials. The solid rubber used in European application No. 0 097 394, for example, is stereospecific 1,4-butadiene rubber in which at least 40% of the unsaturated side chains are in the 1,4-configuration. Unfortunately, the use of solid rubbers of this type is likewise attended by serious disadvantages in terms of practical application. Although the requisite bond strengths and high low-temperature flexibility can be obtained, the adhesives are difficult to apply because of their high viscosity. However, if the viscosity of the adhesives is adjusted to a value suitable for practical application, the result is unsatisfactory strength of the bonds and unsatisfactory rheological properties of the adhesives. The adhesive becomes stringy and runs off from the joints which is also undesirable. In addition, pregelability at comparatively low application temperatures could not be achieved with the rubber-based adhesives disclosed in EP-A 0 097 394 using solid rubbers, such as solid polybutadiene.
In addition, conventional vulcanization systems (hardener systems) could not be used at all with the solid/liquid rubber mixtures disclosed in the prior art. Also, the use of vulcanization accelerators, such as dibenzothiazyl disulfide or diphenyl guanidine, led to the disadvantage that pregelability was only possible at temperatures above 160.degree. C. and to the further disadvantage that, at pregelling temperatures of above 160.degree. C. and hardening temperatures above 200.degree. C., bubbles were formed in the hardened adhesive to the considerable detriment of adhesion.